Multi-layer polypropylene film structure and method of forming the same

Abstract

A multi-layer polypropylene structure of low COF also having antistick characteristics over a wide temperature range and a method of forming the same.

What is claimed is: 
     
       1. An oriented multi-layer polypropylene film structure comprising coextruded layers of: (a) a base layer comprising polypropylene of comparatively high stereoregularity; and   (b) a skin layer comprising a polyolefin of comparatively low stereoregularity on at least one surface of (a), said skin layer containing a combination of finely divided silica, a silicon oil and a polymer incompatible with said skin layer polyolefin, said incompatible polymer having a melting point of greater than 150° F. above the melting point of the skin layer polyolefin.   
     
     
       2. The structure of claim 1 wherein said incompatible polymer is a member selected from the group consisting of poly(methylpentene), polycarbonate, polytetrafluoroethylene and nylon. 
     
     
       3. The structure of claim 2 wherein said skin layer is a member selected from the group consisting of an ethylene-propylene random copolymer and ethylene-propylene-butylene terpolymer. 
     
     
       4. The structure of claim 3 wherein said skin layer is an ethylene-propylene random copolymer with an ethylene content of about 2 to about 7 wt. %. 
     
     
       5. The structure of claim 4 wherein said ethylene-propylene random copolymer has an ethylene content of about 3-5 wt. %. 
     
     
       6. The structure of claim 3 wherein said incompatible polymer is present in from about 1,000-6,000 ppm. of said skin layer. 
     
     
       7. The structure of claim 3 wherein said silicon oil is dimethylpolysiloxane having a viscosity of about 20,000 to 3,000,000 centistokes. 
     
     
       8. The structure of claim 7 wherein said siloxane is present in from about 0.3 to 0.8 wt. % of said skin layer. 
     
     
       9. The structure of claim 3 wherein said silica has a mean particle size of from about 0.5 to 5 microns. 
     
     
       10. The structure of claim 9 wherein said silica is present from about 0.05 to 0.5 wt. % of said skin layer. 
     
     
       11. The structure of claim 3 also containing an antislip proportion of an amide of a water-insoluble monocarboxylic acid having from about 8 to 24 carbon atoms wherein said amide is present in the precursor resin of said base layer in from about 300 to 400 ppm, and present in said skin layer in up to about 2000 ppm. 
     
     
       12. The structure of claim 11 wherein said silicone is dimethylsilicone having a viscosity of from 20,000-30,000 centistokes and present in from about 0.3 to 0.8 wt. % of said skin layer, said silica has a mean particle size of about 0.5-5 microns and is present in from about 0.05 to 0.5 wt. % of said skin layer and said incompatible polymer is poly(methylpentene) present in from about 1000-6000 ppm of said skin layer. 
     
     
       13. The structure of claim 12 wherein the skin layer is an ethylene-propylene random copolymer. 
     
     
       14. The structure of claim 13 wherein the coefficient of friction of the surface of (b) is at least as low as 0.25. 
     
     
       15. The process for preparing an oriented multi-layer polypropylene structure comprising melt coextruding a combination of: (a) a base layer comprising polypropylene of comparatively high stereoregularity; and   (b) a skin layer comprising a polyolefin of comparatively low stereoregularity selected from the group consisting of an ethylene-propylene copolymer and an ethylene-propylene-butylene terpolymer; on at least one surface of (a), said skin layer containing a combination of finely divided silica and a silicon oil; and also containing a polymer incompatible with said skin layer polymer, said incompatible polymer having a melting point of greater than about 150° F. above the melting point of said skin layer polymer; and orienting the structure.